The present invention relates to display devices with a display panel including pixels which are arranged in two dimensions, each pixel being constituted by an element capable of controlling transmittance and reflection of light, and light sources for use with the display devices.
The moving-image-display quality (moving-image quality) of a typical LCD (Liquid Crystal Display) is inferior to that of a CRT (Cathode Ray Tube). This is regarded as a result of slow response speed of the liquid crystal in used.
For the purpose of solving this problem, Journal of the Japanese Liquid Crystal Society (Vol.3, No.2, 1999, pp., 99-106) describes an attempt to improve moving-image quality through an increased response speed of liquid crystal, by adopting a Pi-cell structure whereby a Pi-cell is flanked by optical compensators as shown in FIG. 17.
The paper mentions that a Pi-cell shows an improvement in response speed of liquid crystal over a TN liquid crystal cell: namely, a turn-on time of 1 ms and a turn-off time of 5 ms.
The Pi-cell structure successfully yields a response speed that is fast enough to draw an image in a single frame period. However, the moving-image quality of an LCD with a Pi-cell structure is still inferior to that of the CRT. See FIGS. 18a and 19a illustrating the moving image display on a CRT and a LCD with a Pi-cell structure respectively. The moving images are supposed to be moving in the directions denoted by the arrows.
The paper attributes the quality differences to illuminating characteristics of the CRT and the LCD. FIG. 18b shows the xe2x80x9cimpulse-typexe2x80x9d illuminating characteristics of the CRT whereby pixels emit an impulse of light lasting for a short period of time. In contrast, FIG. 19b shows the xe2x80x9chold-typexe2x80x9d illuminating characteristics of the LCD whereby pixels are hold alight continuously. According to the paper, the degradation of moving-image quality occurs in the LCD, because images in successive fields appear overlapping as a result of the motion of viewpoint.
The paper mentions that the problem is solved by the use of a backlight with impulse-type illuminating characteristics similar to those of the CRT. SID (Society for Information Display), 1997, pp., 203-206, xe2x80x9cImproving the Moving-Image Quality of TFT-LCDsxe2x80x9d, describes a technique to impart impulse-type illuminating characteristics to the LCD (first technique).
According to the first technique, a fluorescent lamp is adopted for use as a backlight of an LCD originally having a hold-type transmittance as shown in FIG. 20b. The fluorescent lamp is flashed as shown in FIG. 20c, using a switching circuit for use with a fluorescent lamp configured as shown in FIG. 20a. The result is impulse-type illuminating characteristics as shown in FIG. 20d (hereinafter, such an LCD will be referred to as an xe2x80x9centire surface flash typexe2x80x9d). The fluorescent lamp in FIG. 20a exhibits illuminating characteristics as show in FIG. 21a when a voltage in FIG. 21b is applied.
The paper describes, as detailed above, a further improvement of moving-image quality of an OCB (Optically Compensated Bend) cell by means of the first technique. A Pi-cell is a type of OCB cell.
The paper further discusses a second technique, whereby the pixels per se of the liquid crystal panel are used as a shutter to impart impulse-type illuminating characteristics to the LCD.
Specifically, a TFT panel 116 is used in which the display section is divided horizontally into an upper screen and a lower screen which are driven by various signals supplied from source drivers 117 and 118 provided to the respective upper and lower screens as shown in FIG. 22d.
The upper and lower source drivers 117 and 118 supplies a black signal and a video signal alternately as shown in FIGS. 22a and FIG. 22c to each pixel of the TFT panel 116. In synchronism with the supply, a gate driver 119 supplies a gate signal shown in FIG. 22b to the TFTs each constituting a pixel of the TFT panel 116. The result is a blanking signal and a video signal being applied within a field period as shown in FIGS. 23b to 23d (hereinafter, such an LCD will be referred to as an xe2x80x9cblack blanking typexe2x80x9d).
According to the second technique, a black display period (interval between RS periods) appears on the hold-type video image in FIG. 23a, moving from the top to the bottom of the panel as shown in FIGS. 23b to 23d. This explains a successful improvement of moving-image quality.
From a viewpoint of flashing a backlight in an LCD module as above, the concept of field sequential color, whereby a color image display is effected by displaying red, green, and blue images in a time series, is similar to the concept of improving moving-image quality.
SID (Society for Information Display), 1999, DIGEST, pp., 1098-1101, xe2x80x9cField-Sequential-Color LCD Using Switched Organic EL Backlightingxe2x80x9d describes a conventional driving method for a field sequential color display. According to the driving method, the device is driven in the time sequence shown in FIG. 24.
Referring to FIG. 24, voltage is applied to a TFT pixel in period (1), response of liquid crystal is awaited in period (2), and an EL (electro-luminescence) backlight is flashed across the screen in period (3). The backlight of this kind of LCD is flashed across the screen similarly to that of the entire-surface-flash-type LCD.
According to the new driving method introduced in the paper, voltage is applied to TFT pixels starting in the top line of the panel and moving down to the bottom line of the panel as shown in FIG. 25. In synchronism with the voltage application to a particular line (however, after a response time of liquid crystal is elapsed), an EL backlight corresponding to that line is flashed.
In prior art example described in the paper, an EL is used as a backlight for use with a field sequential color display; however, a fluorescent lamp may be used instead. In the event, the flashing of the fluorescent lamp should be controlled using the circuit for controlling the flashing of a fluorescent lamp disclosed in Japanese Laid-Open Patent Application No. 11 160675/1999 (Tokukaihei 11 160675; published on Jun. 18, 1999).
FIG. 26 shows the arrangement of a circuit for controlling the flashing of a fluorescent lamp described as a conventional example in the Laid-Open Patent Application.
The circuit for controlling the flashing of a fluorescent lamp, as shown in FIG. 26, includes: high voltage generating means 115 constituted by a DC power source 105 and an inverter 107; and three cold cathode tubes 108, 109, and 110 emitting red, green, and blue light respectively. The cold cathode tubes 108, 109, and 110 are connected in series to switches 111, 112, and 113 respectively. The switches 111 to 113 are each constituted by a high-voltage-resistant bidirectional thyristor which is readily available on the market at a cheap price. By closing one of the switches 111 to 113, a path is established for the high voltage generating means 115 to apply voltage only to the corresponding one of the cold cathode tubes 108 to 110.
This field sequential color technique corresponds to the conventional driving method mentioned above in reference to the SID ""99 paper.
However, in a circuit in FIG. 26 disclosed in the Laid-Open Patent Application, the switches 111 to 113 each constituted by a bidirectional thyristor are not resistant enough to high voltage when they are all open; if the high voltage generating means 115 applies voltage, breakdown takes place in one or more of the open cold cathode tubes 108 to 110, disrupting a complete dark state.
To solve this problem, the Laid-Open Patent Application suggests the use of a novel circuit for controlling the flashing a fluorescent lamp which includes high voltage generating means 114 with an additional switch 106 interposed between the DC power source 105 and the inverter 107 as shown in FIG. 27. When no breakdown is desired in any of the three cold cathode tubes 108 to 110, the switch 106 constituting a part of the high voltage generating means 114 is opened to keep the output level of the inverter 107 below a breakdown voltage, preventing breakdown to occur in all of the cold cathode tubes 108 to 110.
A summary prepared for the 1st LCD Forum of the Japanese Liquid Crystal Society, titled xe2x80x9cDisplay Method of Hold-Type Display Device and Quality of Display of Moving Imagesxe2x80x9d, mentions that quality of moving-image displays on a typical LCD is improved effectively by imparting to the LCD illuminating characteristics which are similar to those of the CRT, i.e., impulse-type illuminating characteristics.
The effectiveness of this method is supported by FIG. 28 showing the relationship between flashing ratios (compaction ratio) and five-level average ratings. The flashing ratio is a period during which a backlight or other illuminating means shines divided by a field period of an LCD or another hold-type display. The five levels average rating represents a result of a subjective evaluation of image quality.
For these reasons, the entire surface flash structure and the black blanking structure have been conventionally employed in LCDs to impart illuminating characteristics which are similar to those of impulse types to them.
However, conventional entire-surface-flash- and black-blanking-type displays still have problems as detailed below.
First, in conventional entire surface flash types of LCDs, display scanning is carried out as shown in FIG. 29; therefore, the display period is equal to a backlight flashing period which is given by equation (1):
Backlight Flashing Period=Field Periodxe2x88x92(TFT Panel Scanning Period+Liquid Crystal Response Period)xe2x80x83xe2x80x83(1)
Equation (1) indicates that entire surface flash types of LCDs have a problem such that the backlight flashing period (display period) is reduced by a value equal to the liquid crystal response speed.
Supposing, for example, that the LCD has a Pi-cell structure, a field period is 16.6 ms, and the response time of the liquid crystal (turn-off time of the Pi-cell) is 5 ms, the backlight flashing period of 8.3 ms (equivalent to a 50% flashing ratio in FIG. 28) is only ensured by the scanning period of the TFT panel of 3.3 ms, which is extremely short compared to those of entire surface hold types of LCDs. The TFT panel in an entire-surface-hold-type LCD has a scanning period which is equal to a single field period at 16.6 ms.
Next, in conventional black blanking types of LCDs, display scanning is carried out as shown in FIG. 30; therefore, the display period is given by equation (2):
Display Period=Field Periodxe2x88x92TFT Panel Scanning Periodxe2x80x83xe2x80x83(2)
Equation (2) indicates that the display period is independent from the response time of the liquid crystal. Accordingly, in black blanking types, the display period is not affected by the response time of the liquid crystal and is longer than those of entire surface flash types by a value equal to the response time of the liquid crystal.
However, black blanking types of LCDs have a problem in CR (contrast) which is inferior to those of entire surface flash types.
In the following, a comparison is made between black blanking types and entire surface flash types on the CR (contrast) in a field period.
The CR of black blanking types is given by equation (3):
CR=(Display Periodxc3x97Bright Display Transmission Ratio)/(Field Periodxc3x97Dark Display Transmission Ratio)xe2x80x83xe2x80x83(3)
In contrast, the CR of entire surface flash types is given by equation (4):
CR=(Backlight Flashing Periodxc3x97Bright Display Transmission Ratio)/(Backlight Flashing Periodxc3x97Dark Display Transmission Ratio)xe2x80x83xe2x80x83(4)
If, for example, the CRs of a black blanking type of LCD and an entire surface flash type of LCD are obtainable respectively from equations (3) and (4), which are rewritten as equations (5) and (6) when substituting 16.6 ms to the field period, 8.3 ms (equivalent to a 50% flashing ratio in FIG. 28) to the black blanking period, the bright display transmission ratio of the TFT display used of 30%, and the dark display transmission ratio of the TFT display used of 0.1%.
CR of Black Blanking Type=(8.3 msxc3x9730%)/(16.6 msxc3x970.1%)=150xe2x80x83xe2x80x83(5)
CR of Entire Surface Flash Type=(8.3 msxc3x9730%)/(8.3 msxc3x970.1%)=300xe2x80x83xe2x80x83(6)
Equations (5) and (6) indicate that the black blanking type has a lower CR than the entire surface flash type.
The present invention has an object to offer a display device such that the backlight flashing period (display period) can be set independently from the TFT panel scanning period, the response time of liquid crystal, etc., so as to ensure an extended operating time of a TFT panel, a display period equal to, or longer than, that of the black blanking type, and a contrast higher than that of the black blanking type.
In order to achieve the object, a first display device in accordance with the present invention includes:
a display panel with pixels which are arranged in two dimensions, each of the pixels being constituted by an element capable of effecting a display through control of transmittance and reflection of light;
a scanning device for carrying out first scanning on the pixels sequentially in a first direction of the display panel so as to set the pixels to respective display states according to information to be displayed by the pixels; and
an illumination device for illuminating the individual pixels, either with intensity of light which increases and subsequently decreases or for a limited period of time, in synchronism with the first scanning carried out by the scanning device, but only after the first scanning.
The first display device, arranged as above, includes pixels arranged in two dimensions, each of the pixels being constituted by a shutter element controlling transmittance (or reflection) of light. The display device carries out the first scanning (display scanning) so as to set the pixels to respective states sequentially in the first direction (scanning direction) according to information to be displayed by the pixels of the display device, and illuminates the pixels after substantially uniform periods have elapsed since the display scanning.
By determining in this manner from which display state to which display state each element, constituting one of the pixels, change and also in which changing state and during which period the element is illuminated, a uniform tone representation always results according to a desired display state without having to wait for the transmittance or reflection state of the element to light to completely change.
Therefore, illuminating periods can be determined independently from the change speeds (response speeds) regarding state change of the elements constituting the pixels.
The illuminating period is determined, for example, depending on how close the illuminating period brings the illuminating characteristics of the pixels in the display device to the impulse type, and as a result, how much the illuminating period improve the display quality of moving images.
During periods that are not designated as illuminating periods, the pixels in the display device do not need to be completely dark, but only have to emit light with a reduced intensity than during illuminating periods to improve moving-image quality.
For example, the illuminating device may control the illumination so that intensity of light illuminating pixels in synchronism with the first scanning exceeds intensity of light illuminating other pixels within a response time in which the pixels completely change the display states thereof.
A second display device in accordance with the present invention includes:
a display panel with pixels which are arranged in two dimensions, each of the pixels being constituted by an element capable of effecting a display through control of transmittance and reflection of light;
a scanning device for carrying out first scanning on the pixels sequentially in a first direction of the display panel so as to set the pixels to respective display states according to information to be displayed by the pixels; and
an illumination device for illuminating the individual pixels with intensity of light which increases and subsequently decreases in synchronism with the first scanning carried out by the scanning device, but only after the first scanning,
wherein:
the scanning device carries out second scanning on the pixels sequentially in the first direction so as to initialize some of the pixels which have changed the display states thereof in the first scanning; and
the illumination device controls the illumination so as to reduce the intensity of light in the first scanning in synchronism with the second scanning carried out by the scanning device.
By carrying out reset scanning as the second scanning to set the pixels to a dark state approximately at the end of the illuminating period which follows display scanning as the first scanning, the second display device in accordance with the present invention sets the pixels in the display device to be dark during periods that are not designated as illuminating periods.
In a case of carrying out reset scanning following display scanning, by lowering intensity of light in each display area of the display device independently from the others approximately at the reset scanning, the reset scanning can be carried out without reduction -in contrast.
Further, the illuminating device may control the illumination so as to vary the intensity of light or illuminating period in synchronism with the first scanning according to the information to be displayed by the pixels.
In other words, the illuminating device may vary the intensity in each display area of the display device according to the information on the pixels in that display area after the first scanning (display scanning)
By varying the intensity of light illuminating each display area of the display device according to the information on the display area in this manner, the display area is set to a maximum luminance which is most suited to the data according to which an image is displayed in the display area.
Further, by varying the maximum luminance for each display area, contrast can be improved, for example, by effecting a white display in a display area and a black display in another display area.
Apart from the control of illumination so that the intensity of light is reduced in the first scanning in synchronism with the second scanning carried out by the scanning device, an illuminating device may also control the illumination so as to illuminate the pixels for a limited period of time during the first scanning in synchronism with the second scanning carried out by the scanning device.
The following light sources are applicable in the display device arranged as above.
A first light source in accordance with the present invention is applicable in any one of the first to third display devices above, and includes:
n elongated light sources (n is a positive integer) disposed in a second direction which is perpendicular to the first direction; and
switches, which are connected in series with the elongated light sources, for controlling turning on/off of the elongated light sources;
wherein,
m flash circuits (m is a positive integer smaller than n) cause the n elongated light sources to flash through the control of the switches.
The light source may be arranged so that it includes another switch, which is interposed between the flash circuits and a power supply device for use with the flash circuits, for controlling connecting/disconnecting of power supply from the power supply device.
Alternatively, the light source may be arranged so that the number, m, of the flash circuits is determined so as to satisfy mxe2x89xa7n/1
where 1 is a positive real number representing a ratio of a field period to a maximum flashing period of the elongated light sources.
In this case, the number of flash circuits can be reduced by the value, nxe2x88x92m, which allows the light source to have a simplified overall arrangement and be reduced in dimensions.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.